FIG. 1 shows a relationship between the classification of electromagnetic waves and wavelengths thereof. First, extreme ultraviolet rays and X-rays will be described with reference to FIG. 1. Extreme ultraviolet rays (EUV) and vacuum ultraviolet rays (VUV) are electromagnetic waves having a wavelength shorter than that of ultraviolet rays in the classification of the electromagnetic waves shown in FIG. 1(a). As can be seen from the comparison of the classification of the electromagnetic waves of FIG. 1(a) with the wavelengths of electromagnetic waves of FIG. 1(b), X-rays indicate electromagnetic waves having a wavelength of 0.001 to 50 nm, wherein soft X-rays indicate X-rays having a wavelength of 0.5 to 50 nm. While a boundary between extreme ultraviolet rays and vacuum ultraviolet rays and soft X-rays is not clearly determined and they are partly overlapped in the classification, extreme ultraviolet rays, vacuum ultraviolet rays, and soft X-rays are electromagnetic waves having an intermediate wavelength of the wavelengths of ultraviolet rays and hard X-rays. Extreme ultraviolet rays, vacuum ultraviolet rays, and soft X-rays have such a property that they have a small amount of transmittancy and absorbed by an air layer. However, since they have a particularly high photon energy, they exhibit a transmittance force which permits them to penetrate the interior of a material such as metal, semiconductor, insulator, and the like from the surface thereof by several hundreds of nanometers. Further, since soft X-rays have such a degree of a photon energy as to be absorbed in inner shell electrons of atoms constituting a material, they exhibit an apparent difference of absorption depending upon elements constituting various materials. This property of soft X-rays is most suitable to the study of various types of materials together with the high resolution thereof. Thus, soft X-rays contributes to the study and development of an X-ray microscope capable of observing living specimens as they are without drying and dyeing them.
Extreme ultraviolet rays (vacuum ultraviolet rays) and X-rays have a high photon energy as compared with that of visible rays and have a high transmittance force to materials. Since extreme ultraviolet rays and X-rays are not refracted in all materials because of the above reason, it is difficult to make a lens. Accordingly, while reflectors are used to converge extreme ultraviolet rays and X-rays and to form images using them, even a metal surface does not always reflect extreme ultraviolet rays and X-rays. However, since the metal surface can reflect extreme ultraviolet rays and X-rays when they are incident on it at an angle close to the metal surface, an optical system making use of oblique incidence cannot be employed.
Thereafter, a great deal of attention was paid to a “multilayer film mirror” capable of reflecting extreme ultraviolet rays (vacuum ultraviolet rays) or X-rays including soft X-rays, which opened a way for developing an optical system in which these rays were incident at near normal angle on an extreme ultraviolet ray and X-ray imaging optical system. An X-ray micrometer making use of X-rays employs the above-mentioned multilayer film mirror. The multilayer film mirror will be described with reference to FIGS. 2(a) and (b).
FIG. 2(a) shows construction of the multilayer film mirror, and FIG. 2(b) shows construction of a reflective film. In FIG. 2(a), the multilayer film mirror is composed of a multilayer film 20 formed on a substrate 10, and FIG. 2(b) shows an example of construction of a multilayer film used for soft X-rays having a wavelength of about 13 nm (photon energy: 97 eV). In FIG. 2(b), the multilayer film 20 is composed of several tens to several hundreds of layers, each including a pair of molybdenum (Mo) and silicon (Si). The multilayer film 20 is attached to the substrate 10 as shown in FIG. 2(a). A normal incidence reflectance of 60% can be obtained by the multilayer film mirror constructed as described above.
FIGS. 3(a) and (b) shows an example of a schematic construction of an X-ray apparatus using the multilayer film reflector shown in FIG. 2(a). In FIGS. 3(a) and (b), the X-ray apparatus is composed of two reflectors, that is, a reflector having the reflective multilayer film 20 attached to the substrate 10 having a concave surface and a hole defined at the center thereof and a reflector having a reflective multilayer film 22 attached to a substrate having a concave surface similarly. Reference symbol L denotes X-rays and the light path thereof.
When X-rays are irradiated toward a body 30 from the left side in FIG. 3(a), the X-rays L are reflected by the multilayer film reflectors 20 and 22, and an enlarged image 35 of the body can be obtained. At that time, the apparatus shown in FIG. 3(a) achieves a role as a microscope as shown in (1) of FIG. 3(b). The image is formed by X-rays the wavelength of which is one several hundredth or less those of visual rays and ultraviolet rays, which can improve the accuracy of even a very fine body making the limit of resolution caused by unsharpness due to diffraction to one several hundredth or less in principle. The above technology is further grown to the development and study of an X-ray telescope of high accuracy, which contributes to the investigation of the origin of the Milky Way and structures of supernovas by the observation of soft X-rays generated from ultra-high temperature plasmas.
Further, when X-rays are irradiated toward the body 35 from the right side in FIG. 3(a), the X-rays L are reflected by the multilayer film reflectors 22 and 20 so that a reduced image 30 of the body comes out. At that time, the apparatus shown in FIG. 3(a) is constructed as an exposing apparatus for executing micro-focusing and reduction as shown (2) of FIG. 3(b). Competition for developing an X-ray multilayer film mirror for a reduced projection exposure optical system is carried out internationally mainly by United States and Japan to use the X-ray multilayer film mirror as a central component of a next-generation ultra LSI manufacturing apparatus.
As described above, the application of the X-ray multilayer film mirror to various fields is expected not only by industrial circles but also by academic circles.
These X-ray multilayer film mirrors must be provided with a wavefront accuracy of at least one eighth or less a wavelength to obtain an imaging performance. To achieve this value, however, it is indispensable to finally develop a method of measuring and correcting an wavefront error at a wavelength of X-rays being used, in addition to the developments of a method of measuring and controlling an accuracy of shape of a spherical substrate, a method of forming a multilayer film, which has a high reflectance and applies no distortion to a substrate, on the substrate, a method of holding an imaging mirror without distortion, a method of adjusting the imaging mirror, and the like.
In particular, the method of correcting a wavefront aberration which is definitely important to the determination of a final imaging performance is encountered with difficulty because an amount of correction is the order of nanometers. At present, an adaptive optics (compensation optical) system for minutely deforming a substrate at an accuracy of nanometers by driving a piezo element and the like, and a method of applying a thin film to the surface of a substrate or ion etching the substrate are proposed.
For example, there is a trial for adaptively correcting a shape of a reflector by an actuator. This trial will be explained by a wavefront aberration correcting apparatus shown in FIG. 4. As shown in FIG. 4, the wavefront aberration correcting apparatus corrects a wavefront by correcting a shape of the multilayer film mirror 20 by applying force to the substrate 10 by an actuator 60 attached to the substrate 10 of a reflector. In the correcting apparatus, soft X-rays L passing through a pinhole 110 are introduced to the reflector by a beam splitter 120 and reflected by the multilayer film mirror 20. In the above construction, when a knife edge 130 is inserted into the light path of the soft X-rays L passing through the beam splitter 120, a shape of a mirror surface can be measured by analyzing an image projected onto a two-dimensional detector 150 with a computer 160. The shape of the reflector is corrected by operating the actuator 60 by a control circuit 170 based on a result of the measurement.
However, these methods are encountered with a great deal of difficulty because they are inevitably required to measure and control a very minute amount of 1 nm or less to geometrically and optically control a reflection surface in principle.